High-energy laser power has value for numerous uses, including industrial applications for material processing and a number of defense applications. Lasers capable of providing energy in the kilowatt (kW) range include CO2 lasers, direct diode lasers, diode-pumped solid-state laser devices such as bulk solid-state lasers, fiber lasers, and disk lasers.
The direct diode laser offers a number of advantages, including relatively high efficiency. However, when using conventional spatial combination techniques, the spatial brightness available from this laser type is constrained, making direct-diode lasers less suitable for high-energy applications such as metal-cutting, for example.
A number of approaches have been employed for combining laser beams from multiple laser elements in order to achieve higher brightness and power output. One approach uses coherent beam combining (CBC) which requires coherence of the individual beams. CBC methods are workable, but constrained due to the need to have phase tightly controlled among multiple laser elements. Another approach combines two laser sources having different polarization states; however, this method is generally limited to two sources. Yet another approach uses wavelength beam combining (WBC). WBC allows the combination of light from laser elements at different wavelengths, using some types of optical systems.
Recent improvements in the use of on-chip gratings or monolithic integrated gratings have helped to provide improved wavelength stability that is useful for high brightness direct-diode lasers. With these devices, the laser wavelength is stabilized without the requirement for optical feedback using an external grating. This type of development helps to make wavelength beam combining applications more promising for scaling the output laser power derived from such lasers.
The simplified schematic diagrams of FIGS. 1A through 1E show methods for wavelength-beam combining that can be used with lasers at different wavelengths in general, including those with or without on-chip gratings. FIG. 1A shows combination of three or more laser sources LS1, LS2, and LSn using dichroic optics. A first dichroic beamsplitter 10 transmits light at wavelength λ1 from laser LS1 and reflects light of wavelength λ2 from laser LS2 onto an optical axis OA. A second beamsplitter 12 transmits light of wavelengths λ1 and λ2 and reflects light of wavelength λn from laser LSn to form a combined output beam 18. As shown in FIG. 1A, one or more additional lasers can be added to the serial arrangement of lasers LS1, LS2, . . . LSn in order to add additional wavelengths to form combined output beam 18.
FIG. 1B shows wavelength beam combining using a lens 14 and a grating 16 in a transmissive embodiment for forming output beam 18. FIG. 1C shows conventional wavelength beam combining using lens 14, with grating 16 in a reflective embodiment.
FIGS. 1D and 1E show wavelength beam combining using free-space optics to direct the combined output beam 18 into an optical fiber 20 or other type of light guide. FIG. 1D uses one or more lenses 14 to combine the laser beams. FIG. 1E uses a parabolic reflector or multiple curved reflective surfaces 22 to form output beam 18.
The conventional approaches shown in simplified form in FIGS. 1A-1E can be used to combine light from multiple laser elements, but, in practice, suffer from a number of shortcomings. Solutions using individual dichroic filters, for example, may result in a bulky optical system. Solutions using refractive or transmissive optics can focus the light from multiple sources, but have high etendue that increases with the number of sources. These solutions are not ideally suited for combining light from multiple sources and directing this light through an optical fiber, for example. Solutions that use gratings require proper spacing and alignment and tend to become too bulky to handle more than a modest number of laser elements.
The beam emitted from a laser element such as a semiconductor diode laser has a highly asymmetric width-to-height (width:height) aspect ratio. In addition, the set of beams emitted from an array of diode lasers are in parallel, so that the output light can be difficult to couple efficiently to an optical system in order to achieve desired levels of optical energy. Thus, it can be seen that there is need for a wavelength beam combining solution that provides a compact and efficient way to combine multiple beams of multiple wavelengths for scaling brightness and power.